I'm not a gigantic SFF (science fiction/fantasy) reader, but for Robin McKinley's work I'll make plenty of exceptions. She won the Newbery for The Hero and the Crown years ago, and her other books are just as good. It's not just the characters and the writing, it's that she is able to create an entire world in every book, a world that is so complete without being too otherworldly (no bizarre robots or spaceships here). Like Rowling has done with the Harry Potter books, she has been able to create a separate space for her characters, a space in which every detail has been thought out and imagined; even if the storyline doesn't revolve around those details, the depth is there.
I can't tell you what it's about, and you shouldn't read the jacket of the book either. Just take me at my word and get it. Make yourself a gigantic cup of cinnamon tea and sit down with the book on your lap and work through it page by page. You'll be glad.